It has been proposed to create a form of hot spring by pumping water through hot strata fractured by drilling and explosive blasting. Whilst this certainly works, it is not very efficient due to the need to have two spaced-apart bore holes, one to supply relatively cold water and the other to recover it after passage through the hot strata. Also it is necessary to use considerable energy in circulating the water, not just through the strata, but also through the great lengths of pipe needed to reach the latter.
It has been proposed that these problems can be at least mitigated by using a single bore hole provided with two tubes assembled in concentric relation, with one tube being used to supply cold water and the other used to recover it. This certainly avoids the need to drill a second bore hole, but it creates an even bigger problem due to the absolute necessity to have a very efficient heat exchanger at the bottom of the bore hole, since the heat must now be recovered from that relatively limited portion of the hot strata in the immediate vicinity of the bottom of the tube assembly. Where, for example, water is circulated through the latter using the single bore hole system the overall efficiency is quite low.
There is still the problem of the energy needed to circulate the water.